Stdcheck.com states fda-approved hiv 4th generation antigen/antibody (duo) test is 97-99% accurate after at least 21 days post exposure and will remain 99% conclusive after 12 weeks. is this true? i took it at 31 days post exposure am i good?

According to the instructions, HIV testing should be confirmed after 12 weeks. Since you tested negative at 31 days post exposure a confirmatory test at 12 weeks has a 99% chance of having the same result. Unfortunately 1% become positive at this point, so it is important to retest.

Yes, you are likely safe but not 100%. Most people, if infected, will develop antibody which can be detected between 15-20 days by 4th generation test in addition to positive p24 antigen but this super-sensitive may still miss 1-3% when test is done @ 31 PE. The false negative result falls to 1% (99% sensitivity) if test is done after 12 weeks. No medical test has 100% sensitivity/specificity.

See below: Early HIV infection is characterized by markedly elevated HIV RNA level. Adding HIV RNA level testing to 4th generation Antigen/Antibody test significantly increase the chance of detection and the accuracy. Some authority recommends RT-PCR test for HIV RNA detection.